1. Field of the Invention
This invention relates to lamp dimming circuits and, more particularly, to dimming circuits that control the brightness of lighting equipment while reducing acoustic noise generated by the equipment.
2. Description of the Prior Art
Control of lighting intensity is desirable in many applications, including theater, sound stage, television lighting and architectural applications. In such applications it is quite common for there to be a plurality of lamps at different locations and, in many instances, banks of such lamps in specified positions. Typically, a plurality of modules each comprising dimming circuits are utilized to achieve gradations of light intensity in each grouping of lighting equipment and thereby obtain special lighting effects.
Dimming control systems in the past have utilized silicon controlled rectifiers (SCR), for example, solid state AC relays utilizing antiparallel connected SCR's to achieve dimming control. Such prior art control circuits have operated on the principal of operating at firing intervals that are a multiple of the line frequency typically 120 Hz to thereby achieve control of the power supplied to the incandescent lamps by controlling the portion of each cycle of the power source that is delivered to the load (phase control). Using phase control, only a fraction of the power available during each cycle is supplied to the incandescent lamps to thereby achieve the ability to dim the output to any level from no dimming (100% light output) to 100% dimming (no light output).
Such dimming control circuits are operable and acceptable for the reason that the human eye integrates the flicker in light intensity and does not detect the rapid "on/off" operation of the lamps when operated at less than full brightness. Prior art SCR control circuits have been characterized by the use of an inductor in series circuit relationship between the source of electric power and the solid state relay. Such circuits, however, have also been characterized by a relatively high amount of acoustic output from the filaments of the lighting devices during their operation. At a minimum such acoustic noise is annoying, particularly to those in the vicinity of the lamps and, in some instances, particularly in stage and television lighting situations, the noise is of a sufficient intensity as to be unacceptable. In addition, because of the size of the inductance required, such circuits have also been characterized by substantial weight, size and bulk.
It is a feature of the present invention that the dimming control circuit according to the invention achieves substantial reduction in the acoustic output of the lighting devices, particularly in the lower ranges of light intensity output, and a physical arrangement that is smaller and lighter in size.